1. Field of the Invention
The present invention relates to a press-forming method and a press-forming apparatus. More specifically, the present invention relates to a press-forming method and a press-forming apparatus capable of forming a workpiece by a punch and a die in a state where the workpiece is grasped by a blank holder.
2. Related Art
Conventionally, a draw-forming of a workpiece such as a sheet metal panel has been carried out by a punch and a die in a state where an end of the workpiece is grasped by a blank holder. The end of the workpiece grasped by the blank holder is unnecessary in a final product. Accordingly, a trimming process for cutting the end is carried out after a draw-forming process.
Herein, as the portion which is cut away by the trimming process (so-called a waste-margin) increases, the yield of material decreases. For example, in a case of forming a rear floor panel of a vehicle which includes a spare tire pan on a rear side thereof and in which a height position of the rear side end is different from a height position of a front side end and the workpiece is draw-formed in a state where the ends of the workpiece are simultaneously supported by the blank holder, the waste-margin on the rear side end becomes too large. Accordingly, the yield of material is significantly degraded. On this account, a forming technology for reducing the waste-margin and improving the yield of material has been developed.
For example, JP-A-04-178225 discloses a forming apparatus which includes an insert die and an insert punch. The insert die can move in a vertical direction relative to a die. The insert punch is arranged to oppose the insert die and can move in the vertical direction relative to a punch. In this forming apparatus, a draw-forming is carried out in a state where a rear side end of the workpiece is grasped by the insert die and the insert punch and a front side end of the workpiece is grasped by a blank holder preceding the die and the punch. Accordingly, the rear side end of the workpiece can be formed while being pressed and restrained at a height position different from the front side end thereof. As a result, the waste-margin on the rear side end can be reduced.
However, in recent years, from a viewpoint of improving a design and a crash-resistance property, an integration of panels has been promoted and thus the shape of a panel member becomes more complicated. For example, it is necessary to form a rear floor panel 100 of a vehicle, which has a plurality of stepped portions therein, as illustrated in FIG. 1. The rear floor panel 100 includes a spare tire pan 110 provided on a rear side thereof and an convex-concave 120 extending in a vehicle width direction between the spare tire pan 110 and a front side thereof.
However, according to the apparatus of JP-A-04-178225, since only one insert die and only one insert punch are provided, the apparatus can only manufacture a panel member having only one stepped portion. In addition, even if a plurality of insert dies and a plurality of insert punches are provided in the apparatus of JP-A-04-178225, since the workpiece is simultaneously grasped, a stroke length of a die-assembly becomes longer and thus the die-assembly is large-sized.